Silicone release coating composition

ABSTRACT

A release coating composition containing about 50 to about 75 wt. % of an epoxy functional silicone; about 8 to about 25 wt. % of a reactive solvent; about 5 to about 25 wt. % of a reactive diluent; about 6 to about 12 wt. % of a wax-treated silica powder; and, about 2 to about 4 wt. % of a photoinitiator is disclosed. The composition is useful in preparing a low-gloss release coating for use on linerless labels.

FIELD OF THE INVENTION

The present invention relates to a release coating composition forlinerless labels. More particularly, the present invention relates to arelease coating composition for use on linerless labels.

BACKGROUND OF THE INVENTION

Linerless labels are known to have advantages over conventional pressuresensitive labels, which are mounted on a disposable, release-coatedliner. The release-coated liner serves as a support structure for thetransport, printing, and storage of conventional labels. In contrast, alinerless label has a release-coated top face and a pressure sensitiveadhesive-coated bottom face, each applied to a paper substrate.

Linerless labels are wound in a roll con-figuration such that thepressure sensitive adhesive bottom face is in contact with the releasecoated top face. The presence of the release coating on a side oppositethe pressure sensitive adhesive enables the labels to be wound to form aroll, and then subsequently unwound, without the adhesive sticking tothe release coating. The adhesive further prevents the wound labels fromunrolling prematurely.

Conventional methods of manufacturing linerless labels are disclosed in,for example, U.S. Pat. Nos. 5,292,713 and 5,354,588. An exemplary methodincludes the continuous steps of passing a thermally sensitive facepaper through a coating machine and sequentially applying a layer ofeach of the various materials (e.g., release coating, and pressuresensitive adhesive) to the paper substrate, drying/curing the materials,and, thereafter, rolling the formed linerless labels into a roll,wherein the release-coated face on the outside of the rolled labels.

According to U.S. Pat. No. 5,292,713, suitable release coatings includematerials that exhibit low adhesion to the pressure sensitive adhesivesuch that the formed label roll can be unrolled easily (e.g., withouttearing the paper substrate, and without having the release coatingstick to the pressure sensitive adhesive). An example of such a releasecoating composition contains 3 percent, by weight (wt. %) of thecomposition, of a photoinitiator based on the total weight of thecomposition, and 97 wt. % of an ultraviolet (UV) curable silicone basedon the total weight of the composition. Both the photoinitiator and thecurable silicone are commercially available, for example, under thetradenames "UV-9380C," and "UV-₉₃₀₀," respectively, from GE Silicones ofWaterford, N.Y.

Linerless labels offer significant advantages over the conventional,liner-backed labels, most notably the elimination of the liner.Nonetheless, linerless labels have met only limited commercial successbecause such labels suffer from several disadvantages. For example, thepressure sensitive adhesives used in the manufacture of linerless labelsare weak, and fail to adhere to the paper substrate, but ratherpreferentially adhere to the release coating. Furthermore, certainpressure sensitive adhesives can detrimentally effect the papersubstrate under certain environmental conditions over time. Attempts toovercome these and other problems have focused on the use of additionaltie layers that enhance bond strength between the release coating,substrate, and adhesive layers. Other attempts have been directed tomodifying the chemical composition of the various layers.

Investigators previously attempted to resolve one or more of theaforementioned problems associated with linerless labels, but wererelatively unsuccessful. One attempt utilized a formulation containing:(a) 57 to 80 wt. % of a curable epoxy silicone compound; (b) 12 to 32wt. % of isopropyl alcohol; (c) 6 to 8 wt. % of an untreated silicapowder; and, (d) 2 to 3 wt. % of a photoinitiator. However, unacceptablyhigh gloss (i.e., a gloss value of greater than 8.2 at 20° using aTechnidyne gloss meter) resulted from formulations having 6 wt. % orless untreated silica powder.

In order to reduce gloss, the amount of silica was increased. But, theuse of higher amounts of the untreated silica powder resulted in aviscous coating that was difficult to coat. Additional isopropyl alcoholwas added to reduce composition viscosity, but the increase in theamount of untreated silica powder in the composition, coupled with thereduction in amount of epoxy silicone compound, resulted in a curedcoating having an unacceptable amount of dusting attributed to poorlyanchored silica powder. This surplus silica dust accumulates on thethermal printheads, thereby requiring cleaning of the printheads on arelatively continual basis. To help reduce the dust problem, the labelswere perforated. However, perforations add to the cost of manufacturingand are not desirable in many applications.

It would be desirable to provide a release coating that overcomes one ormore of the problems associated with linerless labels. Morespecifically, it would be desirable to provide a release coating that:(a) does not adhere to the pressure sensitive adhesive; (b) does notsignificantly insulate the underlying thermal sensitive layer; and (c)is of a suitable quality (e.g., of low gloss) such that the bar codescan be read via electronic means. Furthermore, it would be desirable toprovide a release coating that exhibits sufficiently low spectral glosssuch that the indicia imaged on the linerless label below the releasecoating can be read easily by suitable electronic means. Still further,it would be desirable to provide a low-gloss release coating thatexhibits improved rub resistance, and provides better press stabilityduring the process of manufacturing the linerless labels and eliminatesthe need to perforate the label.

SUMMARY OF THE INVENTION

The present invention is directed to release coating compositions foruse on thermal direct linerless labels. Accordingly, one aspect of thepresent invention to provide release coating compositions that overcomeone or more of the problems associated with prior release coatings forlinerless labels.

As used here and hereafter, the term "release coating composition" isdefined as a liquid composition that is applied to a label substrate.The term "release coating" is defined as a cured film or layer ofrelease coating composition. A release coating composition of thepresent invention is cured with ultraviolet (UV) radiation to provide arelease coating.

In particular, the present invention is directed to a silicone-basedrelease coating composition comprising: (a) about 50 to about 75 weightpercent ("wt. %") of a UV curable silicone compound; (b) about 8 toabout 25 wt. % of a reactive solvent; (c) about 5 to about 25 wt. % of areactive diluent; (d) about 6 to about 12 wt. % of a wax-treated silicapowder; and (e) about 2 to about 4 wt. % of a photoinitiator.

The release coating composition is prepared by a method that includesthe steps of admixing a UV curable silicone compound with the reactivesolvent, followed by the addition of the reactive diluent and thewax-treated silica powder. The resulting mixture is subjected tovigorous agitation for about 10 to about 60 minutes to form a homogenousmixture. Then the photoinitiator is admixed into the homogeneous mixtureto form the release coating composition.

Another aspect to the present invention is to provide a cured releasecoating having a sufficiently low gloss (i.e., a gloss value of lessthan 8.2 at 20° using a Technidyne Gloss Meter) such that an indiciaimage can be rapidly and accurately read by electronic scanners.

Another aspect of the present invention is to provide linerless labels,and particularly thermal direct linerless labels, having a curedsilicone release coating of the present invention.

Other aspects and advantages of the invention will become apparent tothose skilled in the art from the following detailed description, takenin conjunction with the examples and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a release coating compositioncomprising about 50 to about 75 wt. % of a UV curable silicone compound;about 8 to about 25 wt. % of a reactive solvent; about 5 to about 25 wt.% of a reactive diluent; about 6 to about 12 wt. % of a wax-treatedsilica powder; and about 2 to about 4 wt. % of a photoinitiator.

Preferably, the release coating composition comprises about 55 to about70 wt. % of a UV curable silicone compound; about 10 to about 20 wt. %of a reactive solvent; about 8 to about 20 wt. % of a reactive diluent;about 7 to about 11 wt. % of a wax-treated silica powder; and, about 2.5to about 3.5 wt. % of a photoinitiator.

Most preferably, the release coating composition comprises about 60 toabout 65 wt. % of a UV curable silicone compound; about 12 to about 18wt. % of a reactive solvent; about 8 to about 15 wt. % of a reactivediluent; about 8 to about 11 wt. % of a wax-treated silica powder; and,about 2.5 to about 3.5 wt. % of a photoinitiator.

It has been found that a release coating composition, after curing,provides a release coating having a gloss value below 8.2, and typicallya gloss value of below 7, and usually about 6 to about 7, at 20° on theTechnidyne Gloss Meter. This low gloss value provides 100% scan rates oftwo dimensional indicia by various scanners, like laser and CCDscanners. The gloss value of the present release coating issignificantly below the 8.2 value at which scanners begin to misread barcodes.

The UV curable silicone compound present in the release coatingcomposition has a photoreactive functionality, such as an acrylicfunctionality or an epoxy functionality. One class of curable siliconecompound is the epoxy functional silicones.

An epoxy functional silicone present in the release coating compositionis capable of being cured when subjected to UV radiation. Numerous epoxyfunctional silicone compounds, or epoxy silicones, are availablecommercially. For example, epoxy silicones sold under the tradedesignations UV9400 and UV500A are available from GE Silicones,Waterford, N.Y. UV9400 contains 80-99% by weight of dimethyl, methyl,(2-(7-oxabicyclo(4.1.0) hept-3-yl)ethyl silicone having (dimethyl(2-(7-oxabicyclo(4.1.0)hept-3-yl)ethylsilyl)oxy) terminal groups. UV500Acontains about 10-30 percent by weight dimethyl, methyl,2-(7-oxabicyclo(4.1.0)hept-3-yl)ethyl silicone having (dimethyl(2-(7-oxabicyclo(4.1.0) hept-3-yl) ethylsilyl)oxy) terminal groups. Theepoxysilicone in UV9400 and UV500A has a CAS No. 150678-61-8. UV9300 isanother suitable epoxysilicone (containing 80-99% by weight dimethyl,methyl, 2-(7-oxabicyclo(4.1.0) hept-3-yl)-ethyl) silicone (CAS No.67762-95-2), also available from General Electric.

Additional epoxy functional silicone compounds are available from theGeneral Electric Co. under the trade designations UV9315 and UV9320.UV9315 contains 80-99% by weight dimethyl, methyl,2-(7-oxabicyclo(4.1.0)hept-3-yl)ethyl silicone having dimethyl(2-(7-oxabicyclo(4.1.0)hept-3-yl)ethylsilyl)-oxy terminal groups (CASNo. 150678-61-8). UV9320 contains 80-99% by weight(2-hydroxy-phenyl)propyl, trimethyl-heptyl-3-yl)ethyl,methyl-3-methyl-2- (7-oxabicyclo (4.1.0)hept-3-yl) ethyl-silyl)oxy)silicone having dimethyl siloxy terminal groups (CAS No. 130885-21-1).

Other photopolymerizable silicone compounds are available from GeneseePolymers Corporation of Flint, Mich. For example, photopolymerizablesilicone compounds are sold under the trade designations EXP-29 andEXP-32 silicone fluids. EXP-29 is an epoxy-functionaldimethylpolysiloxane copolymer having a molecular weight of about 5700and the structure: ##STR1## EXP-32 also is an epoxy functionaldimethylpolysiloxane copolymer fluid having a molecular weight of about8300 and the structure: ##STR2## Additional epoxy silicone compounds aredescribed in Koshar et al. U.S. Pat. No. 4,313,988, the disclosure ofwhich is hereby incorporated herein by reference.

A typical epoxy silicone has the structure: ##STR3## wherein m rangesfrom 1 to about 10, n ranges from 5 to about 50, and R is an alkylenegroup having 1 to 3 carbon atoms. Other nonlimiting examples of suitableepoxysilicones include UV-9500 available from GE Silicones, Waterford,N.Y., and PC600 and PC620 available from Rhone-Poulenc, Rock Hill, S.C.

The epoxy functional silicones typically have an epoxy equivalent weight(EEW) of about 300 to about 20,000, and preferably about 500 to about10,000. To achieve the full advantage of the present invention, thephotocurable epoxy silicone polymer has an EEW of about 500 to about5000. Accordingly, the epoxy functional silicone typically is a liquidcompound that is readily soluble in the reactive solvent. The epoxyfunctional silicone compound resins are well known in the art, and thepresent composition is not limited with respect to the identity of theepoxy functional silicone compound present in the composition.

The UV curable epoxy compound also can be a silicone compound containingacrylate groups. Such silicone acrylates are well known in the art andare available commercially, for example, from Goldschmidt ChemicalCorporation, Hopewell, Va.

The present release coating compositions also contain a reactivesolvent. Suitable reactive solvents are miscible with the epoxyfunctional silicone and have a reactive group capable of reacting withan epoxy group. The portion of the reactive solvent evaporates duringcure of the composition, and a portion is cured into, and becomes partof, the release coating. The preferred class of reactive solvents isorganic alcohols.

In accordance with an important feature of the present invention, thereactive solvent has an evaporation rate vs. butyl acetate (butylacetate=1) of less than 1.6, and preferably less than about 0.1 to about1.5. Nonlimiting examples of alcohols having an evaporation rate vs.butyl acetate of less than 1.6 include, but are not limited to, n-propylalcohol, amyl alcohol, isobutyl alcohol, n-butyl alcohol, sec-butylalcohol, cyclohexanol, n-hexanol, n-pentanol, methyl amyl alcohol, andmixtures thereof. An especially preferred reactive solvent is n-propylalcohol, which is commercially available, for example, fromChem-Central, Hamilton, Ohio.

The present release coating composition also contains a reactivediluent. A suitable reactive diluent is a liquid monomer that ismiscible with the epoxy functional silicone and reactive diluent, andthat contains two to four reactive moieties that are capable of reactionwith the epoxy silicone. Typically, the reactive moieties are vinylgroups, but can be epoxy or hydroxyl groups, for example. The reactivediluent typically contains two reactive moieties. One nonlimitingexample of a reactive diluent is the divinyl ether of 1,4-cyclohexanedimethanol, which is commercially available under the tradename"Rapid-Cure CHVE" from International Specialty Products, Bound Brook,N.J. Another suitable reactive diluent is "Rapid Cure DDVE," alsoavailable from ISP.

Other reactive diluents include, but are not limited to, a productavailable from Union Carbide under the trade designation UVR-6110.UVR-6110 contains the difunctional epoxy compound3,4-epoxycyclohexylmethyl-3,5-epoxycyclohexane carboxylate. Othercompounds are, for example, bis(3,4-epoxycyclohexylmethyl)adipate,2-(3,4-epoxy-cyclohexyl-5.5-spiro-3,4-epoxy)cyclohexane-metal-dioxane, adiglycidyl ether of phthalic acid, a diglycidyl ether ofhexahydrophthalic acid, a di-glycidyl ether of bisphenol A, acresol-novolac epoxy resin, other difunctional and multifunctional epoxycompounds, and mixtures thereof.

In accordance with an important feature of the present invention, thesilica powder present in the composition is a wax-treated silica powder.The wax-treated silica powder typically is a synthetic amorphous silicasurface treated with hydrocarbon-type wax. The treated silica powder hasa particle size of about 2 to about 10 microns, a bulk density of about7 to about 15 pounds per cubic foot, is insoluble in water, and has adry white powder-appearance at 25° C. A nonlimiting example of awax-treated silica powder is "SYLOID 7000," commercially available fromW. R. Grace & Co., Columbia, Md. Other suitable treated silicas areSYLOID grades 161, 162, 308, 378, 83, and 169.

The present release coating composition also includes a photoinitiatorto facilitate cure of the release coating composition upon exposure toUV radiation. Suitable photoinitiators include iodonium and sulfoniumsalts. Such photoinitiators are theorized to function by ultravioletlight-initiated release of a strong acid that catalyzes the ring-openingpolymerization of the epoxy groups in the epoxy silicones. Cure occurswhen a mixture of the photoinitiator and the epoxy silicone is exposedto a wavelength of light in the ultraviolet (UV) spectrum. The cureoccurs efficiently in air as well as under an inert atmosphere.

The photoinitiator therefore comprises: (i) an iodonium salt, (ii) asulfonium salt, or (iii) a mixture thereof. The anion of these salts isnot limited, but preferably is a complex anion containing Group Va orVIa elements. Exemplary, but nonlimiting, elements present in the anionsare, for example: boron, phosphorus, antimony, arsenic, and tin.Nonlimiting examples of suitable nonbasic, nonnucleophilic anionsinclude, but are not limited to: BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, SnCl₆ ⁻,SbCl₆ ⁻, BiCl₅ ⁻², ClO₄ ⁻, HSO₄ ⁻, ZrF₆ ⁻², GaCl₄ ⁻, InF₄ ⁻, TiF₆ ⁻²,AlF₆ ⁻² and FeCl₄ ⁻².

Nonlimiting examples of sulfonium salt photoinitiators include thetriaryl sulfonium complex salts, such as phenoxyphenyl sulfoniumhexafluorophosphate; trifluoromethyl diphenyl sulfoniumtetrafluoroborate; triphenyl sulfonium tetrafluoroborate, methyldiphenyl sulfonium tetrafluoroborate, dimethyl phenyl sulfoniumhexafluoroborate, triphenyl sulfonium hexafluorophosphate, triphenylsulfonium hexafluoroantimonate, diphenyl naphthyl sulfoniumhexafluoroarsenate, tritolyl sulfonium hexafluorophosphate, anisyldiphenyl sulfonium hexafluoroantimonate, 4-butoxyphenyl diphenylsulfonium tetrafluoroborate, 4-chlorophenyl diphenyl sulfoniumhexafluorophosphate, tri(4-phenoxyphenyl) sulfonium hexafluorophosphate,di(4-ethoxyphenyl) methyl sulfonium hexafluoroarsenate,4-acetonyl-phenyl diphenyl sulfonium tetrafluoroborate,4-thiomethoxyphenyl diphenyl sulfonium hexafluorophosphate,di(methoxysulfonylphenyl) methyl sulfonium hexafluoroantimonate,di(nitrophenyl) phenyl sulfonium hexafluoroantimonate,di(carbomethoxyphenyl) methyl sulfonium hexafluorophosphate,4-acetamidophenyl diphenyl sulfonium tetrafluoroborate,p-(phenylthiophenyl) diphenyl sulfonium hexafluoroantimonate,10-methylphenoxathiinium hexafluorophosphate, 5-methylthianthreniumhexafluorophosphate, 10-phenyl-9,9-dimethylthioxantheniumhexafluorophosphate, 10-phenyl-9-oxothioxanthenium tetrafluoroborate,5-methyl-10-oxothianthrenium tetrafluoroborate,5-methyl-10,10-dioxothianthrenium hexafluorophosphate, dimethyl naphthylsulfonium hexafluorophosphate, and mixtures thereof. Bis-type sulfoniumsalt photoinitiators, such as bis-(4-(diphenylsulfonio) phenyl)sulfidebis-hexafluorophosphate, for example, also can be used.

Many sulfonium salt photoinitiators are available commercially. Forexample, a preferred sulfonium salt initiator is available under thetrade name CYRACURE UVI-6974 from Union Carbide Corporation of Danbury,Conn. CYRACURE UVI-6974 contains a mixture of triaryl sulfoniumhexafluoroantimonate salts having CAS Nos. 89452-37-9 and 71449-78-0,and is sold as a 50 wt. % solution in propylene carbonate. CAS No.89452-37-9 is (thiodi-4,1-phenylene) bis(diphenyl-sulfonium)hexafluoroantimonate. CAS No. 71449-78-0 is diphenyl(4-phenylthiophenyl)sulfonium hexafluoroantimonate. Another suitable sulfoniumphotoinitiator available from Union Carbide Corporation is CYRACUREUVI-6990. UVI-6990 contains triaryl sulfonium hexafluorophosphate saltshaving CAS Nos. 74227-35-3 and 68156-13-8, and is sold as a 50% solutionin propylene carbonate. CAS No. 74227-35-3 isbis(4-(diphenylsulfonio)phenyl) sulfide bis(hexafluoro-phosphate). CASNos. 68156-13-8 is diphenyl phenylthiophenyl sulfoniumhexafluorophosphate.

Nonlimiting examples of useful iodonium salt initiators include the aryliodonium salts, such as diphenyliodonium tetrafluoroborate,di(2,4-dichlorophenyl)iodonium hexafluorophosphate, diphenyliodoniumhexafluorophosphate, diphenyliodonium hexafluoroarsenate,diphenyliodonium iodide, diphenyliodonium hexafluoroantimonate,4-chlorophenylphenyliodonium tetrafluoroborate,di(4-chlorophenyl)iodonium hexafluoroantimonate diphenyliodoniumhexafluorophosphate, diphenyliodonium trifluoroacetate,4-trifluoromethylphenylphenyl-iodonium tetrafluoroborate,ditolyliodonium hexafluorophosphate, di(4-methoxyphenyl) iodoniumhexafluoroantimonate, di(4-methoxyphenyl)-iodonium chloride,(4-methylphenyl)phenyliodonium tetrafluoroborate,di-(2,4-dimethylphenyl)iodonium hexafluoroantimonate,di-(4-t-butylphenyl)iodonium hexafluoroantimonate, 2,2'-diphenyliodoniumhexafluorophosphate, di(4-methylphenyl)iodonium tetrafluoroborate,di(4-heptylphenyl)iodonium tetrafluoroborate, di(3-nitrophenyl)iodoniumhexafluorophosphate, di(4-chlorophenyl)iodonium hexafluorophosphate,di(naphthyl)iodonium tetrafluoroborate,di(4-trifluoromethylphenyl)iodonium tetrafluoroborate,di(4-methylphenyl)iodonium hexafluorophosphate, diphenyliodoniumhexafluoroarsenate, di(4-phenoxyphenyl)iodonium tetrafluoroborate,diphenyliodonium hexachlorostannate, phenyl-2-thienyliodoniumhexafluorophosphate, diphenyliodonium hexafluorostannate,2,2'-diphenyliodonium tetrafluoroborate, di(2,4-dichlorophenyl)iodoniumhexafluorophosphate, di(4-bromophenyl)iodonium hexafluorophosphate,di(4-methoxyphenyl)iodonium hexafluorophosphate,di(3-carboxyphenyl)iodonium hexafluorophosphate,di(3-methoxycarbonylphenyl)-iodonium hexafluorophosphate,di(3-methoxysulfonylphenyl)iodonium hexafluorophosphate,di(4-acetamidophenyl)iodonium hexafluorophosphate,di(2-benzothienyl)iodonium hexafluorophosphate,bis(4-dodecylphenyl)iodonium hexafluoroantimonate, bis(4-dodecylphenyl)iodonium hexafluoroarsenate, and mixtures thereof.

Many iodonium salt initiators are also available commercially. Apreferred iodonium salt is available from the General Electric Co., NewYork under the trade designation UV9380C. UV9380C contains about 30% toabout 60% by weight bis(4-dodecylphenyl)iodonium hexafluoroantimonate(CAS No. 71786-70-4). Other components of UV9380C are 2-isopropylthioxanthone, C₁₂ and C₁₄ alkylglycidyl ethers (about 30% to about 60%by weight), and linear alkylate dodecylbenzene. The C₁₂ and C₁₄alkylglycidyl ethers are monoepoxy compounds and can be considered asincluded in the photopolymerizable component.

Another useful iodonium salt is available from the General Electric Co.under the trade designation UV9310C. The active initiator component ofUV9310C is about 30 to about 60 weight percentbis(4-dodecylphenyl)iodonium hexafluoroantimonate (CAS No. 71786-70-4).Other components of UV9310C are 2-ethyl-1,3-hexanediol (about 30-60weight percent) and a linear alkylate dodecylbenzene (about 5-10 weightpercent). The 2-ethyl-1,3-hexanediol present in UV9310C is a polyhydroxycompound capable of reacting with the epoxy functionalities and can beconsidered as included in the photopolymerizable component of thecomposition.

Other examples of sulfonium salt and iodonium salt photoinitiators arefound, for example, in Guarnery et al. U.S. Pat. No. 4,250,006;Schlesinger U.S. Pat. No. 4,287,228; and Smith U.S. Pat. No. 4,250,053,the disclosures of which are hereby incorporated herein by reference.

In accordance with another important feature of the present invention,the release coating composition is essentially free of water. As usedherein, the term "essentially free of water" is defined as a compositioncontaining 0% up to about 1.5%, by weight, free water. As used herein,"free water" is defined as water that is not bound or complexed toanother entity, e.g., the water of hydration of an inorganic compound isnot included in the calculation of the amount of free water in thecomposition.

It has been found that compositions containing in excess of about 1.5%by weight water failed to cure properly, which led to sticking, or a"locking up," between the release coating and the adhesive coating whenthe linerless labels were rolled. Accordingly, all ingredients used inthe preparation of a present release coating composition aresufficiently free of free water such that the final composition containsabout 1.5% by weight free water or less, and preferably about 1.0% byweight free water or less. To achieve the full advantage of the presentinvention, the composition contains less than about 0.5% by weight freewater.

The release coating compositions can, if desired, include optionaladditives such as dyes, fillers, pigments, flow agents, antistaticagents, thickeners, thixotropic agents, surface active agents, viscositymodifiers, extending oils, plasticizers, and similar additives known topersons skilled in the art to modify an esthetic or functional propertyof the release coating composition, or the release coating preparedtherefrom. These optional ingredients are included in the releasecoating composition in an amount sufficient to perform their intendedpurposes, typically in amounts of 0 to about 10%, by weight, of therelease coating composition.

In accordance with the present invention, the release coatingcomposition first is prepared. The release coating composition isprepared by simply admixing composition ingredients until homogeneous.Then, the release coating composition is applied as a coating ofpredetermined thickness to a solid substrate. The coated substrate thenis subjected to UV radiation to cure the release coating composition andprovide a release coating.

To illustrate the present release coating composition, the followingrelease coating composition was prepared:

    ______________________________________                                        Example                                                                       Ingredient         Weight %                                                   ______________________________________                                        Epoxy Functional Silicone .sup.1)                                                                62                                                         Reactive Solvent .sup.2)                                                                         15                                                         Reactive Diluent .sup.3)                                                                         10                                                         Treated Silica Powder                                                                            10                                                         Photoinitiator      3                                                         ______________________________________                                         .sup.1)  UV9500, GE Silicones, Waterford, NY;                                 .sup.2)  npropyl alcohol;                                                     .sup.3)  divinyl ether of 1,4cyclohexane dimethanol;                          .sup.4)  SYLOID 7000, W.R. Grace and Co., Columbia, MD; and                   .sup.5)  UV9380C, a photoactive iodonium salt available from GE Silicones

The composition of Example 1 was prepared by adding the epoxy functionalsilicone to a vessel equipped with a stirrer. The epoxy silicone wasagitated, then the reactive solvent and reactive diluent were added tothe vessel, and stirring was continued. Next, the treated silica powderwas added to the vessel. The resulting mixture was stirred at a rapidrate for about 30 minutes until the mixture was homogeneous. Finally,the photoinitiator was added, and stirring was continued for about 15minutes. The resulting release coating composition was anhydrous, andhad a shelf life of about 5 days. It is expected that the shelf life canbe extended to 14 days by omitting addition of the photoinitiator to thecomposition.

A release coating is prepared by applying coating or layer of a releasecoating composition of the present invention on a substrate, then UVcuring the composition. The efficient cure of a release coatingcomposition is related to several interdependent parameters. Theseparameters include the chemical identity and photochemical properties ofthe epoxy functional silicone and the photoinitiator. In addition, thestrength of the UV radiation and the cure time are interdependentprocess parameters that are considered in order to achieve asufficiently cured composition for given curing conditions. Accordingly,a present release coating composition is cured for a sufficient timeunder UV radiation (i.e., about 200 to about 450 nm) to provide a dry,tack-free release coating.

In particular, after the release coating composition has been prepared,the composition is applied onto the substrate by conventional meansknown to those skilled in the art. Common coating methods such as rod,blade, multiroll coaters, gravure, or flexographing can be used. Therelease coating composition preferably is applied at a rate of about 0.4to about 1.2 lbs. of composition per 3,000 ft² of substrate, and morepreferably, about 0.5 to about 0.8 lbs. per 3,000 ft², to provide acoated substrate.

The coated substrate then is exposed to UV radiation, such as thatproduced by a 600 watt Type "H" UV lamp, like Model DRW-1101 UPS-6,available from Fusion, Gaithersburg, Md. Upon exposure to UV radiation,the photoinitiator decomposes to generate a Lewis acid. As known in theart, a Lewis acid effectively initiates polymerization of the epoxyfunctional silicone.

The composition of Example 1, therefore, was applied to a surface ofpaper substrate, i.e., T2062 OPTIMA, available from Appleton Paper Co.,using a flexographic print station in order to prepare a linerlesslabel. The composition of Example 1 was applied to the paper substrateas a film having a coating weight about 0.7 lb./3,000 ft². The coatedsubstrate was cured at ambient temperature (i.e., about 28° C.) andunder an ambient atmosphere (i.e., no nitrogen inerting) by passing thesubstrate through an apparatus having a single medium pressure mercurylamp (Type "H" ) to provide UV radiation. A coating of an adhesive wasapplied to the opposite surface of the paper substrate using standardtechniques.

The resulting linerless labels exhibited excellent release when unwoundfrom a roll. The release coating composition also consistently exhibitedgloss value of about 6 to about 7 at 20° on the Technidyne Gloss Meter(a well-known testing apparatus known to persons skilled in the art),and exhibited 100% scan rates of two-dimensional indicia using CCDscanners.

Such a low gloss provided unexpected benefits for both label users andmanufacturers of the label. The low gloss value of about 6 to about 7 iswell below the 8 to 8.2 maximum value that is required to achieve 100%read rates using electronic scanners. Label users, therefore, have arelatively large safety margin with respect to low gloss and accuratescanning. In addition, prior matte linerless labels requiredperforations in order to eliminate dust accumulation on printheads frompoorly anchored silica. The perforation performs as a mild abrasive toremove dust from the printhead. The present release coating overcomesthe dust problems, and, accordingly, the linerless labels do not requireperforations.

Linerless label manufacturers also realize benefits from the presentrelease coating compositions. In particular, the consistent, low glossvalue eliminates the need to test each batch of labels for a suitablegloss value. In addition, using the reactive solvents disclosed hereineliminates the need to adjust viscosity and percent solids at the pressduring extended runs.

The release coating compositions of the present invention providesignificant advantages over prior compositions in that the presentcompositions: (a) do not adhere to the pressure sensitive adhesive; (b)do not inhibit the ability for subsequent imaging of the underlyingthermal sensitive coating; and (c) are of sufficiently low gloss suchthat the bar codes can be read via electronic means. The presentcompositions exhibit a low spectral gloss such that the indicia imagedon the linerless label beneath the coating are easily and accuratelyread by suitable electronic means, like lasers and CCD scanners, forexample, scanners that operate at a peak response of 750 nm(nanometers). Further, the present compositions exhibit improved rubresistance, and provide better press stability during the process ofmanufacturing linerless labels.

Obviously, many modifications and variations of the invention ashereinbefore set forth can be made without departing from the spirit andscope thereof, and, therefore, only such limitations should be imposedas are indicated by the appended claims.

What is claimed is:
 1. A release coating composition comprising(a) about 50 to about 75 wt. % of an ultraviolet radiation curable silicone compound; (b) about 8 to about 25 wt. % of a reactive solvent; (c) about 5 to about 25 wt. % of a reactive diluent; (d) about 6 to about 12 wt. % of a wax-treated silica powder; and, (e) about 2 to about 4 wt. % of a photoinitiator.
 2. The composition of claim 1 wherein the curable silicone compound is present in an amount of about 55 to about 70 wt. %.
 3. The composition of claim 1 when the curable silicone compound comprises an epoxy functional silicone.
 4. The composition of claim 3 wherein the epoxy functional silicone has an epoxy equivalent weight of about 300 to about 20,000.
 5. The composition of claim 3 wherein the epoxy functional silicone polymer is selected from the group consisting of dimethyl, methyl, 2-(7-oxabicyclo(4.1.0) hept-3-yl)ethyl silicone having (dimethyl (2-(7-oxabicyclo(4.1.0)hept-3-yl)ethyl-silyl)-oxy) terminal groups; dimethyl, methyl, 2(7-oxabicyclo(4.1.0) hept-3-yl)ethyl) silicone; (2-hydroxyphenyl)propyl, trimethylheptyl-3-yl)ethyl, methyl-3-methyl-2-(7-oxabicyclo(4.1.0)help-3-yl)-ethylsilyl)-oxy) silicone having dimethyl siloxy terminal groups; ##STR4## wherein m ranges from 1 to about 10, n ranges form 5 to about 50, and R is an alkylene ground having 1 to 3 carbon atoms; and mixtures thereof.
 6. The composition of claim 1 wherein the curable silicone compound comprises a silicone acrylate.
 7. The composition of claim 1 wherein the reactive solvent is present in an amount of about 10 to about 20 wt. %.
 8. The composition of claim 1 wherein the reactive solvent has an evaporation rate vs. butyl acetate of less than about 1.6.
 9. The composition of claim 1 wherein the reactive solvent comprises an alcohol.
 10. The composition of claim 9 wherein the alcohol is selected from the group consisting of n-propyl alcohol, amyl alcohol, isobutyl alcohol, n-butyl alcohol, sec-butyl alcohol, cyclohexanol, n-pentanol, n-hexanol, methyl amyl alcohol, and mixtures thereof.
 11. The composition of claim 1 wherein the reactive diluent is present in an amount of about 8 to about 20 wt. %.
 12. The composition of claim 1 wherein the reactive diluent is selected from the group consisting of divinyl ether of 1,4-cyclohexane dimethanol, 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane carboxylate, bis(3,4-epoxycyclohexyl-methyl)adipate, 2-(3,4-epoxycyclohexyl-5.5-spiro-3,4-epoxy)cyclohexane-metal-dioxane, a diglycidyl ether of phthalic acid, a diglycidyl ether of hexa-hydrophthalic acid, a diglycidyl ether of bisphenol A, a cresol-novolac epoxy resin, a C₈ -C₁₈ alkyl-glycidyl ether, and mixtures thereof.
 13. The composition of claim 1 wherein the wax-treated silica is present in an amount of about 8 to about 11 wt. %.
 14. The composition of claim 1 wherein the wax-treated silica is treated with a hydrocarbon wax.
 15. The composition of claim 1 wherein the photoinitiator comprises an iodonium salt, a sulfonium salt, or a mixture thereof.
 16. The composition of claim 1 comprising:(a) about 55 to about 70 wt. % of the UV curable silicone compound; (b) about 10 to about 20 wt. % of the reactive solvent; (c) about 8 to about 20 wt. % of the reactive diluent; (d) about 7 to about 11 wt. % of the wax-treated silica powder; and, (e) about 2.5 to about 3.5 wt. % of the photoinitiator.
 17. The composition of claim 1 comprising:(a) about 60 to about 65 wt. % of the epoxy functional silicone; reactivet 12 to about 18 wt. % of the reactive solvent; (c) about 8 to about 15 wt. % of the reactive diluent; (d) about 8 to about 11 wt. % of the wax-treated silica powder; and, (e) about 2.5 to about 3.5 wt. % of the photoinitiator.
 18. The composition of claim 1 wherein the reactive solvent comprises n-propyl alcohol.
 19. The composition of claim 1 wherein the reactive diluent comprises a divinyl ether of 1,4-cyclohexane dimethanol.
 20. The composition of claim 1 wherein the composition is essentially free of free water.
 21. The composition of claim 1 wherein the composition contains less than 1.5% free water.
 22. A release coating resulting from ultraviolet curing of a composition of claim
 1. 23. The release coating of claim 22 having a gloss value of about 6 to about 8 at 20°.
 24. The coating of claim 22 having a gloss value of about 6 to about 7 at 20°.
 25. A label comprising a substrate, and having one surface of the substrate coated with a layer of a release coating, said release coating resulting from curing a release coating composition of claim
 1. 26. The label of claim 25 wherein the label is a linerless, thermal direct label.
 27. A label comprising a substrate and a release coating, said release coating comprising a wax-treated silica powder. 